New procedure for chill casting beryllium composite



United States Patent [72] inventors William .1. Richmond Reading;Leonard B. Griffitls, North Reading; Vernon C. Potter, West Concord,Mass. [2 1] Appl. No. 832,439 [22] Filed l l, 19.69

Continuation-impart of application Ser. No. mamas, .Pendi [45] PatentedDec. 22, 1970 [73] Assignee P. R. Mallory & Co. Inc.

Indianapolis, Ind. seemr flqe [54] NEW PROCEDURE FOR Cl-llLL CASTINGBERYLLIUM COMPOSITE 15 Claims, 1 Drawing Fig.

[52] 11.8. CI 164/68, ,5/ 138; .64/58 [51] Int. Cl 022d 21/02 [50] FieldoiSearch 75/138; 164/61, 68; 75/150 [56] References Cited UNITED STATESPATENTS 1,254,987 1/1918 Cooper 75113 8 1,333,965 3/1920 Fahrenwald...75/138 2,399,104 4/1946 Cooper 75/ 1 38 3,049,421 8/1962 Allen et al.75/ l 38X 3,337,334 8/1967 Fenn, Jr. et al. 75/150 3,485,681 12/ l 969Greenewald, Jr l64/322X ABSTRACT: A process for castingaluminum-beryllium alloys comprising, charging a crucible with analuminum base alloy placed at the bottom of a crucible, placing thereona charge of beryllium in which the beryllium is in discrete chunks,lumps or particles, evacuating the furnace chamber for example to removeoxygen and other impurities from the atmosphere, raising the temperatureof the charge sufficient to melt the aluminum but insufficient to meltthe beryllium, maintaining the aluminum molten for a time sufficient toeffect substantially degassing of the aluminum, changing the vacuum toan atmosphere of inert gas at a pressure below atmospheric, raising thetemperature of the charge sufficient to melt the beryllium for theparticular Be-Al alloy composition being cast, and rapidly transferringthe charge at a temperature at least approximately as low as theliquidus temperature into a mold and rapidly cooling said charge.

ATOMIC PER CENT BERYLLIUM PATENIED mm mm ALUMlNUM-BERYLLIUM PHASEDIAGRAM WEIGHT PER CENT BERYLLIUM O u 9T0 L O m H V m 8 w L. L O II 7 Ln O m L L h 5 H w m M L M O. W L 4 M O- 3 I O H e 5 H B m m L. M O 2 V.W m L m- 5 2 II I mw %5 m 0 NWO O \J K 0 O O. O 0 O0 0 O O O 0 o 0 O N Mm w mm 6 5 4 w 0 dmnEmwEzwp ATOMIC PER CENT BERYLLIUM INVENTORS WILLIAMJ. RICHMOND LEONARD B. GRIFFITHS VERNON C POTTER ATTORNEY diagram.

- NEW PROCEDURE-FOR CHILL INC BERYLLI coMi-osrre This application is acontinuation-in-part of application Ser.

No. 793,398, filed Jan. 23, 1969. g I

Several processes for casting Be-Al alloys have been reported comprisingheatinga charge ofberylliumchunks and aluminum alloy chunks in thepresence of a fluxing agent selected from the group consisting of alkaliand alkaline earth halides to a temperature above the melting point ofberyllium. The flux destroys the beryllium oxide film and enables thecasting of a microstructure of beryllium grains interspersed within analuminum or aluminum alloy matrix.

However, under some casting conditions, particularly where theatmosphere has high humidity, it has been found that some porosity hasbeen observed in the castings.

Furthermore, it would be advantageous to avoid the flux because of itscorrosive nature, including its tendency to attack, crucibles, the fumesit creates on volatilization and to avoid the cost outlay for the fluxmaterial.

Because of the corrosive nature of the flux, beryllium oxide cruciblesare preferred which are expensive. t

Furthermore the microstructure' obtained has .notalways been as fine as,desired and the aluminum matrix has sometimes contained undesirableimpurities.

It is therefore an object of the present invention to provide a processwhich avoidsiinpurities in the aluminum matrix.

It is still another object of the presentinvention to provide a processwhich avoids the use of a halide flux,

It is still another object of the present invention to provide a processwhich permits the use of crucibles less expensive than beryllium oxidecrucibles. I

it is still another object of the presentinvention to provide a processwhich results in a very fine as-cast microstructure.

Other objects will appear from the and drawing in which;

The FIG. is a view of the aluminum-beryllium binary phase In order toachieve the procedure is utilized.

The mold to be used for casting is preferably preheated for example, atemperature within the range of about 700 F. to 1000' FL, is 800 F. fora period of time of about .5 to 3 hours, or longer. Furthermore. othertemperatures and times may be used. Moreover this step maybe eliminated,depending upon the shape being-cast, the cast properties desired andother metallurgical variables. a f a The inside of the mold preferablyis coated with amorphous carbon. This may be done before or during thepreheating, if preheating is carried out. This operation can be carriedout for foregoing objects the following example with an acetylene torchadjusted to a reducing flame following description The remainder of thecharge is aluminum or an alloy of alu' minum. For example, siliconand/or magnesium may be present as alloying elements in the aluminum.One aluminum alloy which may be 'used contains about 0,5 to 1.5 percentmagnesium and about 0.5 to L5 silicon with the balance being aluminum.The aluminum to be used is preferably quite pure, most preferably havinga purity of 99.9.9percent or higher with a maximum content of impuritiesother than alloying elements of about 0.0 1 percent total. It is to beemphasized that other alloying elements instead of, and/or in additionto magnesium and silicon may be desirable for certain properties andcharacteristics of the material. his to be understood that the processof the present invention is to include such additional alloying elementsin the aluminum chargel It should also be pointed out that someberyllium will end up in solid solution within the aluminum matrix, forexample 0.2 to 0.5 percent by weight of the aluminum. Y

in charging the aluminum or aluminum alloy is preferably placed at thebottom of the crucible andthe beryllium on top. The reason for this willbecome apparent hereinafter.

The mold preferably afler preheating as described above is placed in afurnace which has. provisions for evacuations, preferably to as low as150 microns and more preferably to as low as l00 microns, and below.Additionally, the furnace should preferably have provisions for-changingthe evacuated system to a system utilizing an inert gassuch as argon, orhelium (argon is preferred).

The melting crucible is preferably placed on a rotatable axis or otherdevice inside the furnace whereby the crucible can be rotated fromoutside the furnace and molten metal poured or otherwise rapidlytransferred into amold which is situated within the furnace, often belowthe crucible. After placing the mold in the furnace, the furnace isevacuated, preferably to a pressure of below 150 microns, morepreferably below 100 microns. It is believed that the evacuation removesoxygen and other impurities from the atmosphere. in addition, it isbelieved that the vacuum treatment also substantially degasses thealuminum matrix. Heat is gradually applied, for example by means of aninduction coil, or other type of heating such as electrical resistanceheating. The heatis applied at a rate so that the aluminum is melted.After the aluminum is melted, it

or by spraying the mold with an amorphous-carbon.-lt is oftenadvantageous to reheat the mold at-for example 20 to 30 minutes atelevated temperature for example about 700-80 The melting crucible,which may be made of MgO, Ago, or otherrefractory oxide rather thanexpensive BeO, is charged with beryllium in small chunks, lumps orparticles. Preferably the size of the chunks is such that they will notpass through a one quarter inch mesh screen but will pass through threequarter inch mesh screen. However for many applications particulatematerial considerably smaller than one quarter inch may be used withgood results.

The amount of Be in the charge is preferably above 40 per-. cent byweight, more preferably about 68 to '85 percent by weight. It is oftendesirable for the beryllium in the charge to have at least some of thefollowing characteristics. For nuclear applications the cobalt ispreferred to be very low, for examplebe below about 0.1 percent. Otherimpurity elements should usually be less than about 0.1 percent and thetotal 'ofthe other impurities should usually be not more than about Ipercent.

is believed that the vacuum degasses the molten aluminum alloy to asubstantial extent. The beryllium remains substan- The sparking can beobserved, for example, through a window in the furnace. g

If desired, in order to observe the pressure at which degassing'iscompleted, an.alloy containing about 0.5 to 1.5 percent magnesium inaluminum, together with beryllium chunks, lumps or particles may beprocessed through the furnace and the pressure determined at whichsparking" takes place. Then aluminum alloys not containing magnesium maybe processed and the pressure at which TSparking" occurred relied on toindicate when degassing has been substantially,

completed. For example, in one 'fumace a pressure of about? 450 micronssignifies the point where the degassing has taken place. However, it isapparent that the pressure at which degassing has taken place willdepend on such variables as the construction of a particular furnace,the type of vacuum pump used, etc.

After the degassing operation has been concluded, the vacuum is shut offand the chamber is filled with an inert gas such as argon or helium,preferably argon, to a pressure which is preferably at least about 5inches of mercury below atmospheric to prevent additional evaporation ofalloying elements, particularly magnesium and the furnace temperature israised in order to melt the beryllium. The heating is continued untilthe beryllium has melted. For example, it can be observed through thefurnace window when all the beryllium chunks have melted. For example,temperatures above about 1,200 C. preferably above about l,300 C. areusually sufficient to melt substantially all of the beryllium. In somecases if induction heating is used, it is believed that an electricfield may be created around the melt by the heating coil. It is believedthat the melt and therefore the beryllium does not melt quickly. If thisproblem exists it can be solved by turning the heating coil off so thatthere is no longer an upward force acting on theberyllium and theberyllium can go down into the aluminum and be melted.

After beryllium has melted, the liquid is slowly cooled to the pouringtemperature. The pouring temperature must be below the liquidustemperature shown on the Al-Be phase diagram for the particularaiuminum-beryllium composition being used. It will be observed from FIG.1 that Be contents of above 40 weight percent including 68 85 percentare on the Be side of the Be-Al eutectic point. For example, the pouringtemperature may be to about 50 C. below the liquidus. This results inthe formation of beryllium nuclei in the melt. As the temperature islowered below the liquidus, a thermal arrest or a point where no changein temperature with time ocours, is observed. The temperature may beobserved for exampie, from a thermocouple placed in the melting crucibleor with an optical pyrometenA pouring temperature of from about 1,] 85to about l,2l C. has been found to be particularly effective for alloyswithin the 68 to 85 weight percent range.

After the pouring temperature is reached, the charge is poured ortransferred quickly into the mold in order to obtain turbulance. Forexample, this can be done by rotating the crucible about its axis topour the molten metal into a mold located therebelow, or by the use ofpressure or vacuum to quickly transfer the molten metal into the mold.it is believed that this procedure results in a fine microstructurebecause as the liquidus temperature is reached a certain number ofberyllium nuclei are formed in the melt and begin to grow dendrites. 1nthe pouring and/or transferring and/or casting turbulence, the dendritearms break off and form new nucleation sites. It is therefore desirableto pour and/or transfer as rapidly as possible and still avoidsubstantial amounts of metal splashing out of the mold. Thus amicrostructure of fine Be grains in an aluminum matrix is obtained. Forexample, about .1 to .3 millimeter average diameter grain size may oftenbe obtained.

It is important that the casting is cooled very rapidly. The rate ofcooling is dependent upon several variables including the conductivityof the mold and the thickness or mass of the mold. Materials having arelatively high thermal conductivity should be used for the mold.Exemplary materials are as follows: copper, copper alloys such as brass,bronze, iron base materials including carbon and alloy steels, andgraphite or other carbonaceous materials. In the shapes which aredifficult to cast and which are subject to hot tearing, it is usuallypreferred to use graphite or other carbonaceous material for the moldand for any cores which would be used.

Themold alsorpreferably has relatively large thickness or mass. It hasbeen found, for example, that M inch thick mold can be used to formcastings 2 X 4 X /2 inch and a fine microstructure is obtained,'but thethicknessand mass will vary according to the shape and size of thecasting, the cast properties desired and/or how fine a microstructure isdesired. Castings having higher minimum dimensions than one-half inchhave been successfully cast.

Aim if desired-additional heat dissipating features may be employedincluding for example water or other fluid heat extracting mediums uponor within the mold and/or fins or other sha es designed for heatdissipation.

ieldstrengths (0.2 percent offset yields) of about 18.000

to about 26,000 p.s.i. and ultimate tensile strengths of about 28,000 toabout 40,000 psi. have been obtained, with the higher values beingobtained with more rapidpouring and/or transferrates. I

According to another embodiment;of the invention platinum is added tothe melt. Rlatinum is believed, to increase the nucleation rate and thusaid intobtaining a fine grain struc-l ture. The platinum may beadded'jntheelemental form be after the entire charge the charge or,alternatively, it'ma has been melted for example by th use of a strandwhich is guided into the melt from outside .th e furnace.' The amount ofplatinum added is for example front about 0.05 to about 0.5 percent byweight of the total charge preferably 0.1 to 0.4 percent by weight. Ifplatinum is added to the charge, the pouring and/or transfer rate doesnothave to be quite so rapid as if platinum were not to be used.

l. A method for casting aluminumberyllium alloys com: prising, charginga crucible with an aluminum or analuminum base alloy placed at thebottom 'of acrucible, placing thereon a charge of beryllium in which theberyllium is in discrete lumps, chunks or particles evacuating thefurnace chamber,

raising the temperature of the charge sufficient to melt the.

content of the charge is above. at least about 40 percent by weightberyllium.

3. A method according to claim 1 in which during the degassing visibleevolution of the particles occurs.

4. A method according to claim 1 in which the molten charge is up to 50C. below the liquidus prior to transferring the charge to a mold.

5. Amethod according to claim lin which the aluminum charge alsocontains an element selected from the group consisting of magnesium,silicon and mixtures thereof.-

6 A method according to claim l'in which the mold is preheated. I i g7,. A method according to claim 1 in which an addition of platinum ispresent in the molten charge at the time of transferring the charge to amold.

8. A method according to claim -l in which the furnace is evacuated atleast as low as 150 microns.

9. A method according to claim 1 in which the inert gas atmosphere is atleast 5 incheszof mercury below atmospheric.

10. A method according to claim 8 in which the charge is heated to atemperature above about l,2000 C.

11. A method according to'claim 3 in which inception of sparking is usedto determine the point when the atmosphere of the furnace is changedfrom a vacuum to an inert gas.

12. A method according to claim 10 in which the charge is heated to atemperature above about l,300 C;

13. A method according to'claim'3 in which the charge. containsmagnesium. Y I

14. A method according to claim'l2 in which the charge is cooled to atemperature'of about l,l to l,2l5 C. prior to transferring the charge toa mold.

IS. A method according to claim 2 in which the-beryllium content isabout 68 to about 85 percent by weight.

